Nest predation by cowbirds and its consequences for passerine demography.

Abstract

Brood parasitism by brown-headed cowbirds (Molothrus ater) reduces reproductive success in many passerines that nest in fragmented habitats and ecological edges, where nest predation is also common. We tested the hypothesis that parasitism and predation are often linked because cowbirds depredate nests discovered late in the host's nesting cycle to enhance future opportunities for parasitism. Over a 20-year study period, brood parasitism by cowbirds was a prerequisite to observing marked inter-and intraannual variation in the rate of nest failure in an insular song sparrow (Melospiza melodia) population. Nest failure increased with the arrival and laying rate of cowbirds and declined when cowbirds ceased laying. The absence or removal of cowbirds yielded the lowest nest failure rates recorded in the study. The absence of cowbirds also coincided with the absence of an otherwise strong positive correlation between host numbers and the annual rate of nest failure. Host numbers, cowbird parasitism, and nest failure may be correlated because cowbirds facilitate nest failure rather than cause it directly. However, an experiment mimicking egg ejection by cowbirds did not affect nest failure, and, contrary to the main prediction of the predation facilitation hypothesis, naturally parasitized nests failed less often than unparasitized nests. Higher survival of parasitized nests is expected under the cowbird predation hypothesis when female cowbirds defend access to hosts because cowbirds should often depredate unparasitized nests but should not depredate nests they have laid in. Where female cowbirds have overlapping laying areas, we expect parasitized nests to fail more often than others if different cowbirds often discover the same nests. We suggest that nest predation by cowbirds represents an adaptation for successful parasitism and that cowbirds influence host demography via nest predation.Brown-headed cowbirds (Molothrus ater) are brood parasites of over 220 species of North American passerines and are thought to reduce host productivity, mainly because adults remove host eggs and nestlings usurp host parental care (1, 2). Nest predation also reduces productivity in cowbird hosts, and parasitism and predation have been jointly linked to population declines in several species, including threatened and endangered neotropical migrants (3-9). This link is currently believed to be a coincidental result of the preferences of nest predators and cowbirds for fragmented habitats and ecological edges (10-15).In contrast, we have argued that cowbirds regularly depredate nests that are discovered too late in the host's nesting cycle to be suitable for parasitism, because this enhances future laying opportunities (16,17). Nest predation may also improve a cowbird's ability to synchronize its own laying with that of its host. This is because most common hosts are territorial and, barring renesting dispersal, will renest within 5-7 days of nest failure (1,8,(18)(19)(20)(21)(22)(23). The potential benefits of nest predatio...

Cuckoos, cowbirds and hosts: adaptations, trade-offs and constraints

“…They showed that such host populations can only be maintained in a metapopulation framework with immigration from other, less parasitized areas. But continued exploitation of hosts can have important conservation implications in rare and localized hosts (Rothstein & Robinson 1994;Arcese et al 1996;Trine et al 1998). The effect of parasitism by Brown-headed Cowbirds, for example, is not restricted to reducing the reproductive success of hosts, but can skew host offspring sex-ratios (Zanette et al 2005) and affect host population growth rate significantly (Smith et al 2002).…”

The Brown-headed Cowbird: North America’s avian brood parasite

“…Perhaps as a result, the Brown-headed Cowbird lacks some adaptations possessed by other brood parasites (Mermoz and Ornelas, 2004) including mimicry of host eggs (Rothstein and Robinson, 1998; but see Peer et al 2002), or evidence of specialized nestling adaptations such as directly killing nestmates (Lichtenstein and Sealy, 1998;Peer et al, 2013). Among the adaptations possessed by cowbird are that females have a larger hippocampus to remember where host nests are located (Sherry et al, 2003); laying eggs rapidly and before sunrise to avoid detection by hosts (Scott, 1991;Sealy et al, 1995; see also Peer and Sealy, 1999b); thick eggshells presumably to withstand puncture-ejection by hosts (Picman, 1989); greater pore diameter in their eggshells for increased embryonic respiration and shorter incubation periods (Jaeckle et al, 2012; see also Briskie and Sealy, 1990); removal of host eggs by females in conjunction with parasitism to enhance incubation efficiency (Peer and Bollinger, 1997; and for nutrition (Sealy, 1992); egg puncture and killing nestlings to force hosts to renest providing additional chances for parasitism (Arcese et al, 1996;Elliott, 1999;Hoover and Robinson, 2007;Dubina and Peer, 2013); and possibly forcing hosts to accept parasitism through mafia enforcement tactics (Hoover and Robinson, 2007).…”

Egg puncture allows shiny cowbirds to assess host egg development and suitability for parasitism

“…Shiny cowbirds do not use punctures to reset yellowwinged blackbirds nesting attempts as was suggested for brown-headed cowbirds (Arcese et al 1996;Peer & Sealy 1999). The number of punctured eggs was no higher in long-incubated, non-parasitized nests than the number of punctured eggs at newly incubated parasitized nests, as would be expected if cowbirds were attempting to force their hosts to renest.…”